In an NMR spectrometer probe, a sample is placed in a static magnetic field which causes atomic nuclei within the sample to align in the direction of the field. Transmit and receive coils, which may be combined in a single coil or set of coils, are placed in the probe positioned close to the sample. The transmit coils apply an RF magnetic field orthogonal to the direction of the static magnetic field, perturbing the alignment of the nuclei. The transmit signal is then turned off, and the resonant RF signal of the sample is detected by the receiver coil. The sensitivity of the spectrometer depends on a number of factors, including the strength of the static field, the closeness of the coupling between the RF coils and the sample, and the resistance of the RF coil.
Currently, most commercial NMR spectrometers use RF coils made of a normal metal, such as copper, or a combination of normal metals, although the use of superconductors in place of conventional normal metal for RF coils in NMR spectrometers may become more commonplace. The advantage to be obtained with high temperature superconductor (“HTS”) coils is significant. HTS coils have very low resistance and are operable in high magnetic fields at temperatures achievable with currently readily available refrigeration systems (above 10K). Cooling of RF coils to reduce their resistance has also been suggested to reduce overall coil resistance. In addition, much research has been devoted to the design of coils for maximum sensitivity. For example, to achieve close coupling, coils have been made that include configurations such as solenoids, saddle coils and birdcage coils, all of which have high filling factors. However, the introduction of HTS materials has led to coil designs that further explore the use of planar coil layouts.
Thin-film HTS coils offer design and processing challenges not present with normal-metal coils. First, commonly used high-temperature superconductors are perovskite ceramics, which require a well-oriented crystal structure for optimum performance. Such orientation is extremely difficult to achieve on a nonplanar substrate. Generally, such coils are preferably deposited epitaxially on a planar substrate. This makes the achievement of a high filling factor more challenging. It is also desirable for the coil to be deposited in a single layer of superconducting film, without crossovers. Second, the coil must be able to handle relatively high currents while producing a uniform magnetic field and avoiding distortion of the B0 field of the magnet.
U.S. Pat. No. 5,565,778 to Brey, et al. discloses a number of different configurations of a probe for NMR spectroscopy. Each of these configurations uses a coil having conductors mounted on a planar substrate. The conductors are arranged such that the coil includes at least one interdigital capacitor. That is, interleaved conductors having a constant spacing between them are located on the substrate. Each conductor surrounds a central sample location and lies closely adjacent to at least one other conductor. None of the conductors completely surrounds the sample location on its own, but the conductors are in an alternating arrangement such that adjacent conductors have respective breaks in their conductive paths at different radial positions relative to the sample location. This results in a capacitive configuration that forms a coil surrounding the sample location.
In another recent patent, U.S. Pat. No. 6,556,013 to Withers, planar coil layouts were further refined, and an example of one of these is reproduced in FIG. 1. As shown, the Withers patent describes an oblong conductor layout that allows the magnetic field-generating conductive elements 22 to be closer to the sample volume than the interdigital capacitive elements 24. That is, the capacitors are segregated to the “top” and “bottom” of the coil, and the vertical elements along the coil sides are used as the primary RF magnetic field generating components. Thus, the magnetic field that is generated has a stronger influence on a sample than it would if the same conductors were located in the top and bottom of the coil, which are further away. This arrangement retains the benefit provided by the capacitors, while keeping them away from the sides of the coil, where they would otherwise limit the magnetic field generating capacity of the vertical elements. The vertical elements on each side of the coil are also electrically connected to one another by optional conductive nodes 23.
In both HTS and normal metal coils, coil failure can result during the transmit pulse when operating the coils at their expected high voltages. These failures are thought to be caused by a number of different factors, but typically result in a catastrophic breakdown between some of the relatively narrow coil conductors, and ultimate destruction of parts of the coil. It is thought that minute material defects, contamination and unexpected power surges can trigger arcing between coil conductors, which can have a cascading effect throughout the coil. The incidences of arcing typically occur in the capacitive region of the coil, where high voltages dominate. The arcing in an HTS coil renders parts of the capacitors nonconductive, causing the coil's resonant frequency to rise, often to the point that the coil is no longer usable.